Density and phase see text

R-phrases , S-phrases , , , ,
,

The chemical compound nitrogen dioxide (NO₂) is a reddish or orange/brown gas with a characteristic sharp, biting odor. It is one of the most prominent air pollutants and an insidious poison by inhalation.

Formation and occurrence

---

Nitric oxide (NO), also a common pollutant, oxidizes in air to the dioxide:

2 NO + O₂ → 2 NO₂

NO₂ is generated by the action of nitric acid on a variety of metals, such as copper or silver.

2 HNO₃ + Ag → AgNO₃ + NO₂ + H₂O

"Red fuming nitric acid" owes its red color to the presence of NO₂.

NO₂ is generated in biological settings from decomposition of peroxynitrite (ONOO-), a potent oxidizing and nitrating agent formed from the reaction of nitric oxide with superoxide.

Reactions

---

Nitrogen dioxide exists in equilibrium with its dimer, dinitrogen tetroxide.

2 NO₂ ↔ N₂O₄

The equilibrium favors NO₂ at higher temperatures. Solid NO₂ can be obtained as a solid from NO₂ by very rapid cooling (for example with liquid nitrogen), although it is commonly contains N₂O₄.

At −50 °C the crystals of N₂O₄ are colorless, but honey-yellow at the melting point. The vapour at −10 °C is pale yellow and deepens as the temperature rises

Structure and bonding

---

NO₂ is a radical having a lone - or unpaired - electron, which renders this molecule paramagnetic and is involved in the electronic transitions that give rise to its color. The dimer, N₂O₄, is diamagnetic.

Safety and pollution considerations

---

Nitrogen dioxide is toxic by inhalation. Symptoms of poisoning (lung edema) tend to appear several hours after one has inhaled a low but potentially fatal dose. Also, low concentrations (4 ppm) will anesthesize the nose, thus creating a potential for overexposure.

Long-term exposure to NO₂ at concentrations above 40–100 µg/m³ causes adverse health effects ^*. The most important source of NO₂ are internal combustion engines, which emit nitrogen oxides near people.

This map, depicting results of satellite measurements, illustrates nitrogen dioxide as large scale pollutant, with rural background ground level concentrations in some areas around 30 µg/m³, not far below unhealthful levels. Nitrogen dioxide plays a role in atmospheric chemistry, including the formation of tropospheric ozone.

A recent study by researchers at the University of California, San Diego, suggests a link between NO₂ levels and Sudden Infant Death Syndrome ^*.

See also

---

• Nitrous oxide or N₂O, "laughing gas", a linear molecule, isoelectronic with CO₂ but with a nonsymmetric arrangement of atoms (NNO)
• Nitric oxide or NO, a problematic pollutant, related to CO but with one additional electron.
• Nitrogen dioxide, NO₂, also a serious pollutant, with a bent structure .
• NOx = all of the above in unspecified proportions but tending toward NO₂.

More esoteric nitrogen oxides include N₂O₅ and the blue species N₂O₃.

Oxidized (cationic) and reduced (anionic) derivatives of many of these oxides exist: nitrite or NO₂^-, nitrate or NO₃^-, nitronium or NO₂⁺, and nitrosonium or NO⁺. NO₂ is intermediate between nitrite and nitronium:

NO₂⁺ + e → NO₂

NO₂ + e → NO₂^-

External links

---

• International Chemical Safety Card 0930
• National Pollutant Inventory - Oxides of nitrogen fact sheet
• NIOSH Pocket Guide to Chemical Hazards
• WHO-Europe reports: Health Aspects of Air Pollution (2003) (PDF) and "Answer to follow-up questions from CAFE (2004) (PDF)
• Nitrogen Dioxide Air Pollution — A summary for non specialists by GreenFacts of the above WHO reports.
• Nitrogen dioxide pollution in the world (image)
• Computational Chemistry Wiki

Nitrogen compounds | Oxides | Bleaches | Pollutants | Smog

Oxid dusičitý | Stickstoffdioxid | Dioxyde d'azote | Diossido di azoto | Stikstofdioxide | 二酸化窒素 | Oxid dusičitý | 二氧化氮